Bar percussion instruments such as marimbas, xylophones, vibraphones and glockenspiels generate sound in two ways. They have a row or rows of tone bars supported on a frame. The tone bars comprise a solid material, including but not limited to wood, metal, fiberglass, graphite or composites of these. Hanging beneath and in sonic communication with the tone bars are corresponding rows of hollow tubular resonators. When a musician batters a tone bar, that mechanical energy is transferred to air in the resonator, creating a standing wave, and an audible tone. The resonator amplifies and concentrates the fundamental frequency generated by the tone bar.
Each resonator tube has a proximal end nearest the tone bars, and an opposing distal end. Sometimes the distal end points toward the floor and sometimes it points toward the ceiling, or toward the player, or to a left or right side. The proximal end is open and hangs just beneath, but does not touch, its corresponding tone bar. The distal end is closed, capped or plugged, defining a length. This length is important, as it determines the frequency of the sound it generates when its tone bar is struck. When a tone bar is struck, it generates a standing wave that passes through the resonator tube. When the tone bar and its resonator tube are in tune, they produce a strong, clear and rich note. This is called the fundamental tone. To achieve the fundamental tone, resonator tubes must each be cut to precise lengths. Instrument manufacturers use a precise mathematical formula to determine the length of the tubes. For example, the length of a tube necessary to generate middle C is equal tolength=c/4v where c is the speed of sound in m/s and v is the frequency in Hertz. For air, c=343 m/s. For middle C, v=262 Hz. Therefore, to make a resonator for the middle C key, the tube should be cut to:343/(4)(262)=0.327 m or 32.7 cm
The manufacturer then closes, caps or plugs the tube at the distal end. This ensures that this standing wave is the correct frequency. Some manufacturers permanently cap the tube at the prescribed length. Others sell open tubes, with separate caps that the musician can insert into the distal end and lock into place. The separate cap has the advantage of letting the musician adjust the effective length of the resonator tube, and therefore its frequency.
Most prior art resonator caps or plugs are made from metal or plastic. Plugs function exactly like what the name implies. They are a slightly smaller diameter than an inner diameter of the tube and can be inserted into the open distal end of the resonator tube, then expanded in place using internal circular leaf springs, radial cutouts in the plug itself, foam rubber and/or mating threads. Resonator plugs are commonly solid metal, or a tube of metal within a tube of metal. In addition to the metal, the plugs may also contain a layer of foam, or a filter, to muffle certain overtones produced by the tone bars and to enhance the overall fundamental tones of the instrument. Expanding the plug in this way creates a seal which traps the column of air within the tube at the desired length.
Another justification for having insertable plugs sold separately is that they make it easy for the musician to compensate for changes in ambient temperature and humidity. It is true that in general, temperature affects how musical instruments sound. Warmer air causes wood and metal to expand, decreasing pitch. Cooler air causes wood and metal to contract. The reader should keep in mind that tone bars are solid wood or metal. Resonator tubes are hollow wood or metal. Decreasing the temperature causes both the bar and the walls of the tube to contract. This is true for both metal and wood. When the walls of the tube contract, the space within the tube increases. When the tone bar contracts, it produces a higher or sharper pitch sound. At the same time, the walls of the resonator tube also contract. More space inside the tube actually creates a lower or flatter pitch sound.
One skilled in the art would know that while the weather does affect each part of the instrument, the net effect is that the instrument remains in tune. The contraction or expansion of the various parts stays in proportion. Experience has shown that there is no measurable benefit from having a separate part to tune the resonator, at least not due to air temperature. The temperature does affect the instrument, but only negligibly. Manufacturers of these caps try to sell musicians on the ability of the caps to compensate for short term changes in temperature when in fact this does not measurably affect tuning.
The most significant cause of tone bars and tubes growing out of tune is wear and tear on the tone bars. Over time, the constant battering, setting up and taking down the instrument chips away, cracks, dents, scrapes and damages the wood or metal. A bar which has lost mass or is deformed will sound differently than it once did. Frequent changes in temperature can also permanently change the quality of the wood or metal. Repeatedly battering the bar with a mallet can also heat up the bar and cause it to temporarily expand. It then cools down. These constantly alternating temperatures have long-term effects on tuning. These variables affect the tuning much more so than temporary and small changes in ambient temperature. My resonator cap is detachable and can be tuned to a tone bar simply by tightening or loosening the cap against the bearing edge of the resonator.
Other prior art resonator caps have different problems. Marimba resonators produced in Mexico and other Central American countries have a membrane attached to small holes at the distal end of the tube. Usually this membrane is some sort of intestine skin and is often adhered to the tube with glue or wax. This creates a permanent buzzing effect that a musician may or may not want. This membrane style cap is thin and prone to damage.
My integrated resonator cap solves these, and other, problems. It requires no separate parts and is fully tunable. A flexible, yet durable, percussive playing surface seals the distal end of the resonator tube. The cap is crafted from a membrane that is so strong that it can be battered like a drum. This is like having a whole other instrument to play. Optionally, secured against this membrane is an amplifying tube. In a preferred embodiment, the amplifying tube is a cone. Other shapes of amplifying tubes are possible and within the scope of this invention. These other shapes include but are not limited to cylinders, coils and prisms. When a musician strikes a tone bar, this creates a sound wave. The wave travels through the resonator tube. The energy from the wave vibrates the membrane and transfers the energy to the cone, amplifying the sound wave beyond what the resonator tube would do alone. The cone is secured to the membrane so that even if the cap is turned upside down, the cone remains held against the membrane.
I am not aware of any marimbas, xylophones or other bar percussion instruments that have a flexible membrane which simultaneously creates a percussive playing surface and a platform for an amplifier, and further, tunes the resonator. My system allows a musician to manipulate the resonator's sound in ways that were not possible before. Musicians can use this system as a traditional resonator tube, an amplified resonator tube with a new fundamental tone, as an independent, miniature drum, or as a combined resonator and mini-drum. My invention creates infinitely many new creative playing, performing and composing options for this class of instruments.